The presence of large deposits of oil shale in the semiarid high plateau region of the United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is, in fact, a misnomer, it is neither shale nor does it contain oil, it is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents such as U.S. Pat. Nos. 3,661,423; 4,043,597; 4,043,598 and 4,192,554 as well as pending patent application Ser. No. 246,232 filed Mar. 23, 1981, by Chang Yul Cha, entitled TWO LEVEL, HORIZONTAL FREE FACE MINING SYSTEM FOR IN SITU OIL SHALE RETORTS. Each of these applications and patents is assigned to Occidental Oil Shale, Inc., assignee of this application, and each is incorporated herein by this reference.
These patents and applications describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale, wherein such formation is explosively expanded to form a stationary fragmented permeable mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort, or merely as a retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone in the retort and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone through the fragmented mass. In the combustion zone oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceious materials to produce heat, combustion gas and combusted oil shale. By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbons, and a residual carbonaceous material.
The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products together with water produced in or added to the retort collect at the bottom of the retort and are withdrawn. An off gas is also withdrawn from the bottom of the retort. Such off gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition and any gaseous retort inlet mixture that does not take part in the combustion process.
U.S. Pat. Nos. 4,043,597; 4,043,598; and 4,192,554, disclose methods for explosively expanding formation containing oil shale toward horizontal free faces to form a fragmented mass in an in situ oil shale retort. According to such a method a plurality of vertically spaced apart voids of similar horizontal cross section is initially excavated one above another within the retort site. A plurality of vertically spaced apart zones of unfragmented formation is temporarily left between the voids. Explosive is placed in each of the unfragmented zones and detonated to explosively expand each unfragmented zone upwardly and/or downwardly towards the void or voids above and/or below it to form a fragmented mass having an average void volume about equal to the void volume of the initial voids. Retorting of the fragmented mass is then carried out to recover shale oil from the oil shale.
U.S. Pat. application Ser. No. 246,232 discloses a method for explosively expanding formation containing oil shale towards a horizontal free face to form a fragmented mass in an in situ oil shale retort. According to such a method, a void having a horizontal cross section similar to the horizontal cross section of the retort being formed is initially excavated. A plurality of vertically spaced apart zones or lifts of unfragmented formation is left above the void. Explosive is placed in each of the unfragmented lifts and detonated for explosively expanding such zones towards the void to form a fragmented mass in the retort having an average void volume about equal to the void volume of the initial void. The overlying lifts can be expanded towards the void in a single round or a plurality of rounds. Retorting of the fragmented mass is then carried out to recover shale oil from the oil shale.
As used herein the term "lift" means a generally horizontally extending layer of the subterranean formation that is explosively expanded in a single round with no more than a short delay interval between explosive charges used for explosively expanding such a lift. In this context a lift can be explosively expanded either upwardly or downwardly. A delay interval is considered short when adjacent explosive charges interact to provide greater explosive expansion than can be provided by explosive charges that do not interact. An exemplary short time delay is about one to two milliseconds per foot of spacing distance between adjacent explosive charges. When forming an in situ oil shale retort by explosively expanding two or more overlying zones or lifts into an underlying void in a single round, it has been suggested that it is desirable to detonate explosive charges in successive lifts with as small a time delay as possible between successive lifts. A typical suggestion is that the time delay between successive lifts should be on the order of about one millisecond of delay per foot of burden of the later lift being explosively expanded. This technique may not result in optimum fragmentation and expansion of the formation when explosively expanding formation towards a limited void. Even if expansion of a subsequent lift could keep a preceding lift from overexpanding, optimum distribution of void fraction in the resultant fragmented mass may not be obtained. The subsequent lift or lifts may not have adequate room to fragment and expand, leaving regions with low permeability in the fragmented mass.
It is desirable to provide a technique for explosive expansion of successive lifts towards an underlying limited void which assures adequate fragmentation to avoid an excessive amount of large particles of formation that may not be completely retorted and to assure reasonably uniform void fraction distribution in the fragmented mass formed by explosive expansion. This helps avoid regions of excessively low permeability which would interfere with uniform gas flow or prevent retorting of portions of the fragmented mass. It is also desirable to minimize the quantity of explosive required for explosive expansion and minimize overbreak in adjacent unfragmented formation.